1. Field of the Invention
This invention relates to the detection of an out-of-range condition in thermistors used in a climate control apparatus and, more particularly, to detection of an out-of-range condition in either or both of two thermistors in a two-speed heat pump.
2. Description of the Prior Art
The typical heat pump is comprised of four primary components of equipment including a compressor, a flow restriction valve, and two heat exchangers. One of the heat exchangers is customarily referred to as the heating or cooling coil, depending on the operating cycle, and is ordinarily positioned in the building structure within duct work employed for directing the circulating room air. A fan is typically positioned adjacent the heating and cooling coil to induce the circulating air to flow over the coil and through the duct work. The second heat exchanger (commonly known as the discharge coil), the compressor, and the flow restriction valve are typically contained within a single housing unit positioned on the exterior of the building structure to be heated or cooled. A discharge fan is provided adjacent the discharge coil to induce heat exchange between the discharge coil and the ambient air. These various heat pump elements are connected in a closed series loop by tubing carrying a liquid refrigerant. The direction of flow of refrigerant in the closed system is easily reversed by a series of valves. This ready reversal of refrigerant flow direction allows the heat pump to be quickly switched between its cooling and heating cycles.
The heat pump operates most efficiently when the temperature difference between the interior and exterior environments is less than approximately 50 degrees Fahrenheit. When this temperature difference increases out of the efficient operating range of the heat pump during the heating cycle, auxiliary heat is required, and usually provided by the combustion of fossil fuel in an oil or gas fired furnace or by resistance electric heat.
Current two-speed heat pump products have two thermistors; one senses the ambient temperature and the other senses the outdoor coil temperature. A number of heat pump functions are dependent upon the readings from these two thermistors. The outdoor coil thermistor (or defrost thermistor) is used to both initiate and terminate the defrost cycle of the heat pump. The value of the ambient thermistor is referenced in energizing the crankcase heater, determining the outdoor ICM (integrated control motor) speed, second-stage latching (allowing, under certain conditions, for second stage or high speed cooling to satisfy a call by the indoor thermostat for first stage cooling), the interfacing of the heat pump with the furnace, the heating switch-over speed point for setting the compressor level, defrost initiation, and the outdoor thermostat function for auxiliary heat.
In the prior art it was possible to determine if one of the thermistors was shorted out or frozen in an open condition. However it was not possible to determine if a thermistor was out of the proper resistance range. This could lead to possible problems; for example if the ambient temperature thermistor is out of range and reads low, the system will not initiate the defrost cycle when it should. However no error code would be provided to indicate the nature of the problem and, as a result, the control which should provide the error code may be misdiagnosed as being faulty rather than having the out-of-range condition of the thermistor being recognized and corrected.
In prior art patents, for example U.S. Pat. No. 5,097,288 to Akao et al. and U.S. Pat. No. 4,995,240 to Barthel et al. two thermistors are used in order to sense the temperature of different elements and control portions of a system accordingly (in Akao et al. in an image forming apparatus and in Barthel et al. to control refrigerant flow in an air conditioning system). However nowhere in the prior art is the problem of one of the thermistors being or becoming out of range considered.